Heterogeneity of Leaf-Level Photosynthesis as Seen Through the Lens of Ozone, Soybean, and Chlorophyll Fluorescence Imaging
Leaf-level heterogeneity of photosynthetic CO2 assimilation occurs naturally in plants under stressed and non-stressed conditions and has been attributed in the past to stomatal heterogeneity. Variation in the biochemistry of mesophyll cells has been cited as another potential cause of photosynthetic heterogeneity, but there has been no work done to investigate the effects of biochemical heterogeneity. The biochemistry of C3 leaf photosynthesis under steady-state conditions is widely characterized in vivo by the Farquhar et al. (1980) model to interpret gas exchange data and in turn to model steady-state CO2 assimilation in leaves, canopies, landscapes and the globe. Understanding the effects of heterogeneity in the key biochemical parameters Vc,max (the maximum Rubisco carboxylation rate) and Jmax (the maximum rate of whole chain electron transport) on the validity of the Farquhar et al. (1980) model could be crucial to improving the existing models of plant productivity. One particular inducer of leaf-level photosynthetic heterogeneity is tropospheric ozone (O3), a phytotoxic air pollutant whose rising global background concentrations have been widely recognized as a problem affecting both natural communities and agricultural systems, especially plant species such as soybean (Glycine max). This thesis aims to further the present understanding of the effects of leaf-level biochemical heterogeneity in photosynthesis and advance the current ability to measure and analyze this heterogeneity. Modulated chlorophyll a fluorescence imaging (CFI) coupled with gas exchange was used to test the hypothesis that O 3 induces differential leaf-level spatial effects on photosynthesis in soybean depending on its exposure regime (acute vs. chronic). Spatial analytic techniques derived from the field of remote sensing were adapted to quantify the leaf-level spatial heterogeneity of photosynthetic processes within the leaf. The fluorescence imaging data revealed that the pattern and character of spatial damage differed between acute and chronic [O3] treatments, implying that acute [O3] treatment is likely invalid as an effective proxy for the chronic damage that occurs in the field situation. A computer modeling approach was used to conduct a theoretical study to determine if leaf-level heterogeneity in photosynthetic biochemical parameters (such as that induced by O3 stress) would have significant impact on the Farquhar et al. (1980) model and the current paradigm of gas exchange interpretation. The theoretical study determined that the effect of biochemical heterogeneity would depend on the scale of variation and the extent to which V c,max and Jmax varied in concert across the leaf. However, there is no established method to estimate the natural leaf-level variation and spatial patterns of Vc,max and Jmax. Two new methods were developed from the equations of Farquhar et al. (1980) to allow mapping of V c,max and Jmax modulated chlorophyll imaging in such a way that would be independent of heterogeneity in stomatal conductance. These methods were validated using whole-leaf data and then applied to healthy and O3-damaged leaves to quantify their leaf-level heterogeneity in Vc,max and Jmax. Combining this data with the previous theoretical study, it was determined that the application of the Farquhar et al. (1980) model is affected very little by within-leaf heterogeneity in these biochemical parameters and in stomatal conductance, even under the extreme effects of acute O3 exposure.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign